1 - Nuclear Sciences and Applications - IAEA

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IAEA-CN-50/E-HI-3 641
FIG. 3. Same as Fig. 2 for ne = 10 H cm~ 3 .
Some information on the power deposition profile can be inferred from the
sawtooth activity. In fact, the time derivative of the peak electron temperature,
T'e, immediately after the sawtooth collapse, is proportional to the difference
between the central power density input, Pj, and the stationary power loss, PL.
= Assuming that PL is proportional to Pj we can write: P^RF/P T'(RF)/T'
Since
(0H).
T'(RF) = 66 eV-ms~' and T'(0H) = 40 eV-ms"
obtain
1 , we
Pi(RF/P>(OH) = 1-7- Since the peak Ohmic power density, Pn, obtained from the
neoclassical conductivity, is ~5 W-cm" 3 , we obtain a value of 3.5 W-cm" 3 for the
central RF power density. This figure suggests that the RF power deposition region
is well inside the half-radius, even if only 18% of the launched power is accessible
to the plasma core. This evaluation could be affected by a change in Pn, caused by
the RF. But as we have already noted, the total Ohmic power does not change during
the RF phase, and the effective ion charge, Zeff, shows only a small increase (from
1.3 to 1.45).
3. THE ROLE OF ACCESSIBILITY
As is well known [7], only power components with ni larger than a critical
value, nnc, can propagate through the plasma. Since nnc increases with the density,
it is evident that with increasing density the plasma core becomes inaccessible to
waves with higher phase velocity. This implies that the energy of the fast electrons
generated by the waves should decrease with the plasma density.
This has been studied by looking at the electron cyclotron emission in a spectral
region, between 1.3 «ce and 1.6 coce, where thermal emission and reabsorption are
absent.